In an electrophotographic printing system, a photoconductive member is typically charged to a substantially uniform potential to sensitize the surface thereof. The charged photoconductive member is then exposed to a light image. Exposure of the charged photoconductive member selectively dissipates the charges thereon in the irradiated areas This exposure process produces or "records" an electrostatic latent image on the photoconductive member corresponding to informational areas within the document being printed. After the electrostatic latent image is recorded on the photoconductive member, the latent image is developed by bringing developer material into contact therewith. Toner particles from the developer material are attracted to the electrostatic latent image at a development station to develop a toner powder image that is subsequently transferred from the photoconductive member to a substrate sheet. The toner particles on the substrate sheet are then heated to permanently affix the powder image thereon.
The foregoing generally describes a typical single color electrophotographic printing system. With the advent of multicolor electrophotography (including highlight-color (typically one color plus black) and full-color output), it is desirable to use single-pass architectures that comprise a plurality of image forming stations (including tandem or image-on-image architectures) arranged in seriatim as described, for example, in U.S. Pat. No. 5,300,962 to Genovese, issued Apr. 5, 1994. The single-pass architecture offers a significant potential for increased throughput and improved image quality.
As is well known, image registration is an important and difficult problem in a xerographic color image printing system. Generally, multiple color separations are produced on a common photoreceptor or intermediate member so as to coincide with one another and produce a full color image, for example, as found in the Xerox.RTM. 4850 or 4890 Highlight Color Printing Systems. Subsequently, the color image is transferred to paper and fixed thereon. Alternatively, in lieu of an intermediate belt or photoreceptor a copy sheet transport or conveyor may be employed so that the color separations are transferred directly to the delivery medium.
In order to deliver quality images, strict specifications are imposed on the accuracy with which the various color separations that compose the individual images are exposed. In highlight color systems, the juxtaposition accuracy, often referred to as image registration, typically must be maintained within a 125 .mu.m (micrometers) range and preferably is less than 75 .mu.m, where the range represents the diameter of a circle that would encompass all homologous color dots. In process color systems the required juxtaposition accuracy is on the order of 25 .mu.m or less and preferably about 10 .mu.m. Furthermore, some imaging techniques require registration accuracy on the order of about 15 .mu.m for pictorial information.
Color printers that employ registration marks produced by each of the constituent colors in juxtaposition with each other enable correction of lateral and longitudinal relative position, skew and magnification. The marks may be machine readable, and data may be processed to measure registration errors for the purpose of automating registration error correction. Measurement of the position of each of the registration marks may be accomplished by illuminating the marks and employing a lens to collect the diffusely reflected light to image the reflection on photodetectors or photodetector arrays. The illumination may be in the visible wavelength or at near infrared (IR) wavelength. In order to reliably detect the position of the registration mark, the diffuse reflection from the registration mark must be significantly different from its background. It is desirable therefore, to achieve high contrast for bright or black belts and for image output terminals (IOTs) for which the first printed color has high or low diffuse reflectivity.
The present invention is directed toward methods and apparatus for employing registration error distributions measured with sufficient spatial resolution by sensors, and from such registration error distributions, compensating for photoreceptor velocity so as to eliminate motion errors that are typically repeatable in amplitude and phase.
Heretofore, a number of patents and publications have disclosed methods and apparatus for adjusting the position of raster scanning beams in response to photoreceptor motion, the relevant portions of which may be briefly summarized as follows:
U.S. Pat. No. 4,903,067 to Murayama et al., issued , discloses a marking system with a detector for measuring alignment errors and then mechanically moving individual color printers to correct misalignment.
A number of patents describe methods for adjusting the position or intensity of individual light beams so as to compensate for changes in the photoreceptor motion, including: U.S. Pat. No. 4,514,739 to Johnson et al., issued Apr. 30, 1985; U.S. Pat. No. 5,319,393 to Genovese, issued Jun. 7, 1994; U.S. Pat. No. 5,287,125 to Appel et al., issued Feb. 15, 1994.
U.S. Pat. No. 5,287,160 to Dastin et al., issued Feb. 15, 1994, discloses a color printer employing various motion components having circumferences that are either multiples or submultiples of a predetermined pitch distance Any systematic error due to motion of the components is repeated identically for each color separation, thereby eliminating misregistration for the plurality of color separations.
U.S. Pat. No. 5,287,162 to de Jong et al., issued Feb. 15, 1994, teaches a detection system for applying chevron marks deposited at sequential printers so as to detect motion error and correct therefor.
U.S. Pat. No. 5,043,744 to Fantuzzo et al., issued Aug. 27, 1991, discloses a raster output scanner (ROS) wherein the velocity of the imaging member is controlled as a function of the position of the imaging beam, and where the tilt of the beam is adjusted relative to the imaging member.
U.S. Pat. No. 4,807,156 to Parisi, issued Feb. 21, 1989, teaches an improved control for adjusting the size of an image printed by a printer, where the length (process direction) of an image on the photoreceptor may be adjusted using a cycle stealing component. The cycle stealing component periodically, in response to a predetermined algorithm based upon desired magnification, delays the signal used to drive polygon rotation in the raster scanner.
U.S. Pat. No. 5,1 53,644 to Yang et al., issued Oct. 6, 1992, discloses a dual-mode correction system for compensating for both vibratory and photoreceptor motion errors in a xerographic printing apparatus. An encoder is employed to sense both photoreceptor speed and relative vibration between the phtotoreceptor and the imaging device. The signal produced by the encoder is then processed to obtain high and low frequency error signals. The low frequency signals are applied to correct photoreceptor motion error, while the high frequency signals are used to control a beam wobble correcting element in the imaging device.
U.S. Pat. No. 4,837,636 to Daniele et al., issued Jun. 6, 1989, teaches a xerographic copying/printing machine wherein the photoreceptor speed is sensed using a plurality of equally spaced holes along an edge thereof, the holes being sensed by a charge coupled device in response to light shining through the holes. The monitoring system employed is used for timing the recording member in order to generate clock signals for use in maintaining the operating speeds of related machine components in synchronization with the recording member despite changes in recording member speed.
U.S. Pat. No. 5,278,587 to Strauch et al., issued Jan. 11, 1994 discloses a method of registering images on a photoreceptor belt so as to form sequential images in a single pass. A pair of transversely aligned belt holes, in conjunction with associated light detectors, are used to position the ROS exposed images on the photoreceptor belt.
U.S. Pat. No. 5,278,625 to Charnitski et al., issued Jan. 11, 1994 teaches a LED-based, single-pass imaging system employing an improved lateral registration system. An LED print bar control system establishes an initial registration for each print bar and subsequently maintains the registration within tolerance by detecting relative lateral motion between the print bar and the photoreceptor.
In accordance with the present invention, there is provided a multiple-image registration apparatus in a printing system for synchronizing the output of a plurality of imaging stations with respect to a member moving relative to the imaging stations, comprising:
a servomotor for driving the member; PA1 a first imaging station, located along the periphery of the member, for producing a first transferable image on a surface of the member, said first imaging station producing rasterized lines of the first image at a fixed line frequency on the surface of the member in response to data of a first color separation; PA1 a first velocity sensing means, positioned adjacent the first imaging station and in contact with the member, for sensing the velocity of the member as it passes said first imaging station and producing an electrical signal representative thereof; PA1 a first phase-locked loop controller for receiving the electrical signal from the first encoder and a signal representing the fixed line frequency, said first phase-locked loop creating a servomotor drive signal as a function of the electrical signal and fixed frequency signal so as to cause the servomotor to drive the member in synchronization with said first imaging station; PA1 a second imaging station, located along the periphery of the member downstream from said first imaging station, for producing a second transferable image on the member, said second imaging station producing rasterized lines of the second image in response to a second scanline clock signal; PA1 a second velocity sensing means, positioned adjacent the second imaging station and in contact with the member, for sensing the velocity of the member as it passes said second imaging station and producing a second electrical signal representative thereof; and PA1 a second phase-locked loop controller for receiving the second electrical signal and creating the second scanline clock signal as a function of the second electrical signal so as to cause the second imaging station to produce rasterized lines of the second image on the member in registration with the rasterized lines of the first image in response to data of a second color separation. PA1 driving, with a servomotor, a photoresponsive member in a process direction relative to the plurality imaging stations; PA1 producing, at a first imaging station, located along the periphery of the photoresponsive member, a first transferable image on the surface of the photoresponsive member, the first image comprising rasterized lines at a fixed frequency; PA1 sensing the speed of the photoresponsive member as it passes the first imaging station and producing an electrical signal representative thereof; PA1 creating, as a function of the electrical signal and a signal representing the fixed frequency, a servomotor drive signal to drive the photoresponsive member in synchronization with the production of the rasterized lines of the first transferable image at the first imaging station; PA1 producing, at a second imaging station located along the periphery of the photoresponsive member and spaced apart from said first imaging station in the process direction, a second transferable image on the surface of the photoresponsive member, the second image comprising rasterized lines produced in response to a scanline clock signal; PA1 sensing the speed of the photoresponsive member as it passes the second imaging station and producing a second electrical signal representative thereof; and PA1 creating the scanline clock signal as a function of the second electrical signal to synchronize the second imaging station to produce rasterized lines of the second image on the photoresponsive member in registration with the rasterized lines of the first image. PA1 a first imaging station, located along the periphery of the recording member, for producing a first transferable image on a surface of the recording member, said first imaging station producing rasterized lines of the first image at a fixed line frequency on the surface of the recording member; PA1 a servomotor for driving the recording member; PA1 a first encoder, positioned adjacent the first imaging station and in contact with an opposite surface of the recording member, for sensing the speed of the recording member as it passes said first imaging station and producing an electrical signal representative thereof; PA1 a first phase-locked loop controller for receiving the electrical signal from the first encoder and a signal representing the fixed line frequency, and creating therefrom a servomotor drive signal as a function of the electrical signal and the fixed frequency signal so as to control the speed of the servomotor and thereby drive the recording member in synchronization with said first imaging station; PA1 a second imaging station, located along the periphery of the recording member yet spaced apart from said first imaging station in the direction of travel of the recording member, for producing a second transferable image on the recording member, said second imaging station producing rasterized lines of the second image in response to a scanline clock signal; PA1 a second encoder, positioned adjacent the second imaging station and in contact with the opposite surface of the recording member, for sensing the speed of the recording member as it passes said second imaging station and producing a second electrical signal representative thereof; PA1 a mark detector for generating a mark detection signal upon detection of a mark placed on the surface of the member by said first imaging station; PA1 a second phase-locked loop controller for receiving the second electrical signal and creating the scanline clock signal as a function of the second electrical signal and the fixed frequency signal so as to cause the second imaging station to produce rasterized lines of the second image on the recording member at a spatial separation equal to that of the rasterized lines of the first image; and PA1 a mark detect counter, responsive to the mark detection signal, for delaying the output of the second imaging station for a predetermined period subsequent to receiving the mark detection signal so as to produce the second image on the surface of the member in registration with the rasterized lines of the first image.
In accordance with another aspect of the present invention, there is provided a method for synchronizing the output of a plurality of imaging stations in a single-pass, multi-color printing system comprising the steps of:
In accordance with yet another aspect of the present invention, there is provided a multi-color printing machine having a movable recording member, including:
An object of the present invention is to assure that the photoreceptor drive velocity is locked to the imaging frequency, particularly the rotation speed of a polygon employed in a raster scanning device, and that the velocity can be easily calibrated without regard for photoreceptor drive and encoder roll diameters or tolerances. A second object of the present invention is to employ marks on the photoreceptor, either permanent or xerographically deposited and associated with each image or pitch position, to provide accurate image registration for each color separation. A third object of the present invention is to assure that the raster line rates (frequencies) and the phases of the rasters produced by the second and subsequent imaging stations are in registration with respect to the first image deposited.
One aspect of the invention deals with a basic problem in multicolor image registration, where in order to deliver quality images, strict specifications are imposed on the accuracy with which the various color separations that compose the individual images are registered on a photoreceptor or similar imaging member. This invention is further based on the discovery of a phase-locked loop technique that alleviates this problem. The technique not only synchronizes the imaging of a first color image to the speed of the photoreceptor, but also controls and synchronizes the imaging rate of any subsequent color separations as a function of photoreceptor position (distance travelled) and the imaging rate of the first imaging station so as to assure accurate registration between the first and subsequent scanlines and color separations. The technique described above is advantageous because it does not rely on mechanical or optical mechanisms to alter the position of the subsequent image separations, but handles the alteration by electronically altering the timing of the image creation (e.g., exposure). Accordingly, the technique described accomplishes accurate multicolor image registration without the need for additional mechanical hardware.